Gay rights advocates have filed a court challenge to the government's unusual plan to canvass Australians' opinion on gay marriage next month, while a retired judge said he would boycott the survey as unacceptable.

Gay rights advocates have filed a court challenge to the government's unusual plan to canvass Australians' opinion on gay marriage next month, while a retired judge said he would boycott the survey as unacceptable.

The mail referendum is not binding, but the conservative government will not legislate on the issue without it. If most Australians say No, the government will not allow Parliament to consider lifting the nation's ban on same-sex marriage.

Lawyers for independent politician Andrew Wilkie and marriage equality advocates Shelley Argent and Felicity Marlowe applied to the High Court for an injunction that would prevent the so-called postal plebiscite from going ahead.

"We will be arguing that by going ahead without the authorisation of Parliament, the government is acting beyond its power," lawyer Jonathon Hunyor said.

Prime Minister Malcolm Turnbull said the government had legal advice that the postal ballot would withstand a court challenge.

"I encourage every Australian to exercise their right to vote on this matter. It's an important question," he said.

Gay-rights advocates and many politicians want Parliament to legislate for marriage equality now without an opinion poll, which they see as an unjustifiable hurdle to reform.

"It's just something we've never done in our constitutional arrangements of Australia, and it really is unacceptable," he told Australian Broadcasting Corp.

Mr Kirby would not comment on the legality of the government proceeding with the 122 million Australian dollar (£74 million) ballot without Parliament's approval, but said: "I'm not going to take any part in it whatsoever."

Plebiscites in Australia are referendums which do not deal with questions that change the constitution. Voting in referendums is compulsory to ensure a high turnout and that the legally-binding result reflects the wishes of a majority of Australians.

The government opted for Australia's first-ever voluntary postal plebiscite after the Senate twice rejected 135 million Australian dollars (£82 million) in funding for a conventional plebiscite, in which votes are cast in ballot boxes.

Pollster Martin O'Shannessy said a market researcher would be able to provide a more accurate picture of Australians' attitudes toward gay marriage through telephone polling for less than 1.2 million Australian dollars (£727,550).

The accuracy of the postal ballot could be damaged by low voter response and responses coming primarily from voters with strong views on both sides of the argument, said Mr O'Shannessy, a partner at Sydney market research firm Omnipoll.

"I would say that the process is open to significantly more risk than an opinion survey," he said.

Successive opinion polls show most Australians support gay marriage. But national referendums in Australia rarely change the status quo. Gay rights advocates fear that on an issue that does not directly affect most Australians, a majority might be persuaded to opt against change.